solver means "Solar Version", a weird name with only historical significance now

It has no relation to the verb "to solve".

solver is the directory tree into which header files for inter-module
use and build results (libraries, shared libraries, executables) are
copied ("delivered") during the build, and from where they are then
picked up when constructing an installer (Windows), a RPM or deb
package (Linux) or an app bundle (Mac).